1. Field of the Invention
The invention relates to the manufacture of pistons.
An in-line engine for an internal combustion engine comprises a cylinder block having a number of cylinders of generally circular cross-section arranged in succession along the length of the block; this length being determined by the cooling requirements of the cylinders and the diameters of the cylinders. Any increase in the cylinder diameter, to produce an engine of greater cubic capacity and thus of greater power, will increase the length of the block. Any such increase in the length of the block is also disadvantageous because it necessitates a more expensive crankshaft construction and an increase in the required size of an engine-receiving compartment in an associated vehicle. Alternatively, it necessitates the use of a more expensive "vee" cylinder arrangement.
It is possible to increase the swept volume of the cylinders, and hence increase the power of the engine, by making the cylinders, and their associated pistons, of oval cross-section with the major axis of such a section extending normal to the length of the engine block. As used throughout this specification, a reference to "oval" or to particular forms of oval, such as ellipses, is defined as a reference to an oval where the ratio of the major axis to the minor axis is between 1.1:1 and 3:1, except where the context expressly provides otherwise. The use of oval cylinders or pistons can therefore increase the swept volume of the engine by between 10% and 300% without any increase in the length of the engine block. In addition, the larger cylinder area allows a more favourable disposition inlet and outlet ports and inlet and outlet valves of greater diameter than can be used with generally circular section cylinders and pistons, so improving the breathing of the engine.
2. Review of the Prior Art
In general, pistons and cylinders (whether oval or not) are produced by casting the cylinders and pistons roughly to shape and then by finishing the cylinders and pistons to their final form. In the case of a cylinder, this generally comprises a boring operation using a machine tool followed by a honing operation to give the bore a suitable surface topography to ensure lubricant retention and hence low running friction. In the case of a piston, one common way of finishing is by turning the exterior surface of the piston. It is customary for the piston to be machined with a "barrelled" longitudinal form (i.e. to have a diameter which is a minimum at the end of the piston and a maximum in the centre) and to be slightly non-circular, to allow for expansion effects. Further, the piston may be provided with bearing surfaces in accordance with the teaching of British Patent Specification No. 2079661.
While all these operations have been performed satisfactorily for many years in respect of pistons and cylinders that are right cylindrical or substantially right cylindrical, by the use of conventional machine tools. In general such machine tools cannot cope with pistons and cylinders which are oval, because the required range of tool movement cannot be achieved at speeds which allow the commercial production of cylinders and pistons of significantly oval shape.